Koloboke aims to replace the standard Java collections and streams  with more efficient implementations.  The current version of Koloboke focuses on replacing java.util.HashSet and java.util.HashMap. It provides a complete set of primitive type implementations for each collection. Its able to avoid the expensive boxing/unboxing of primitives and saves memory for boxed primitive objects. It is the fastest and the most memory efficient library implementing hash maps and sets.

Koloboke -   Java Collections till the last breadcrumb of memory and performance

GS Collections is a collections framework for Java. It has JDK-compatible List, Set and Map implementations with a rich API and set of utility classes that work with any JDK compatible Collections, Arrays, Maps or Strings. The iteration protocol was inspired by the Smalltalk collection framework. It provides replacements for ArrayList, HashSet, and HashMap optimized for performance and memory usage.

gs-collections - A supplement or replacement for the Java Collections Framework

Fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and queues with a small memory footprint and fast access and insertion. It also provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files. It has support of bidirectional iterators. It has support of Big arrays and Big Lists with 64 Bit indices.

Fastutil extends the Java Collections Framework by providing type-specific maps, sets, lists and queues with a small memory footprint and fast access and insertion. It also provides also big (64-bit) arrays, sets and lists, and fast, practical I/O classes for binary and text files.

Fastutil - Fast & compact type-specific collections for Java

Colt distribution consists of several free Java libraries bundled under one single uniform umbrella. Namely the Colt library, the Jet library, the CoreJava library, and the Concurrent library. 

 The Colt library provides fundamental general-purpose data structures optimized for numerical data, such as resizable arrays, dense and sparse matrices (multi-dimensional arrays), linear algebra, associative containers and buffer management. The Jet library contains mathematical and statistical tools for data analysis, powerful histogramming functionality, Random Number Generators and Distributions useful for (event) simulations, and more. The CoreJava library contains C-like print formatting. The Concurrent library contains standardized, efficient utility classes commonly encountered in parallel & concurrent programming.

Colt distribution consists of several free Java libraries bundled under one single uniform umbrella. Namely the Colt library, the Jet library, the CoreJava library, and the Concurrent library. It provides support for resizable arrays, dense, sparse matrices, histogramming functionality, Random Number Generators etc.

Colt - Scientific and Technical Computing in Java

Hazelcast is a clustering and highly scalable data distribution platform for Java. It supports Distributed implementations of java.util.{Queue, Set, List, Map}, java.util.concurrency.locks.Lock, java.util.concurrent.ExecutorService, Distributed Indexing and Query support, Dynamic scaling, partitioning with backups, fail-over, Web-based cluster monitoring tool and lot more. 

Hazelcast - In-Memory Data Grid for Java

HPPC provides template-generated implementations of typical collections, such as lists, sets and maps, for all Java primitive types. The primary driving force behind HPPC is optimization for highest performance and memory efficiency. 

HPPC - High Performance Primitive Collections for Java

Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities.
 <ul><li>Bag interface for collections that have a number of copies of each object</li>
	<li>Buffer interface for collections that have a well defined removal order, like FIFOs</li>
	<li>BidiMap interface for maps that can be looked up from value to key as well and key to value</li>
	<li>MapIterator interface to provide simple and quick iteration over maps</li>
	<li>Type checking decorators to ensure that only instances of a certain type can be added</li>
	<li>Transforming decorators that alter each object as it is added to the collection</li>
	<li>Composite collections that make multiple collections look like one</li>
	<li>Ordered maps and sets that retain the order elements are added in, including an LRU based map</li>
	<li>Identity map that compares objects based on their identity (==) instead of the equals method</li>
	<li>Reference map that allows keys and/or values to be garbage collected under close control</li>
	<li>Many comparator implementations</li>
	<li>Many iterator implementations</li>
	<li>Adapter classes from array and enumerations to collections</li>
	<li>Utilities to test or create typical set-theory properties of collections such as union, intersection, and closure</li>
	</ul>

Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities.  It provides collection class like Identity map, Ordered map, Reference map, Composite collections, Bag interface etc.

Apache Commons - Alternative to Java collection

Trove provides fast, lightweight implementations of the java.util Collections API. These implementations are designed to be pluggable replacements for their JDK equivalents. This gap in the JDK is often addressed by using the "wrapper" classes (java.lang.Integer, java.lang.Float, etc.) with Object-based collections. For most applications, however, collections which store primitives directly will require less space and yield significant performance gains. Trove eliminates the need to create Map.Entry wrappper objects for every item in a table and so reduces the O (big-oh) in the performance of the hashtable algorithm. The size of the tables used in Trove's maps/sets is always a prime number, improving the probability of an optimal distribution of entries across the table, and so reducing the the likelihood of performance-degrading collisions. Trove sets are not backed by maps, and so using a THashSet does not result in the allocation of an unused "values" array.

Trove provides fast, lightweight implementations of the java.util Collections API. These implementations are designed to be pluggable replacements for their JDK equivalents. This gap in the JDK is often addressed by using the "wrapper" classes (java.lang.Integer, java.lang.Float, etc.) with Object-based collections. For most applications, however, collections which store primitives directly will require less space and yield significant performance gains.

Trove - High performance collections for Java

Mango is a Java library consisting of a number of iterators, algorithms and functions, loosely inspired by the C++ Standard Template Library. It has support of 12 iterators, 16 generic algorithms and variety of function types. It supports Array Iterator, Bounded Iterator, Chain Iterator, Null Iterator, Reverse Iterator, Selecting Iterator, Singleton Iterator, Skipping Iterator, StringIterator, Tee Iterator, Transform Iterator, Zip Iterator. It supports function types such as Generators, Functions, Binary Functions, Predicates, Binary Predicates.

Mango is a Java library consisting of a number of iterators, algorithms and functions, loosely inspired by the C++ Standard Template Library. It has support of 12 iterators, 16 generic algorithms and variety of function types.

Mango - specialized iterators and functions

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Koloboke - Java Collections till the last breadcrumb of memory and performance